The present invention relates to satellite communications and to earth stations (terminals) for geostationary satellites using xe2x80x9cbent pipexe2x80x9d (analog repeater) transponders. More particularly, the invention relates to small networks (as few as two stations, i.e., a point-to-point link) for satellite communications.
Communications links are commonly implemented with repeaters on above-atmosphere platforms such as satellites. In a typical satellite link, (refer to FIG. 1) forward signals (11) transmitted from earth station A (10) pass through the earth""s atmosphere, are linearly amplified by a constant amount by a transponder in the satellite 15, and transmitted down through another part of the atmosphere to earth station B (12). Earth stations may be equipped to transmit, receive, or both. A reverse signal (13) may be similarly transmitted from earth station B to earth station A. Occasional atmospheric disturbances such as rain may attenuate the signal on the uplink path (i.e., from an earth station up to the satellite), the downlink path (i.e., from the satellite down to an earth station), or both. These events are called xe2x80x9cfadesxe2x80x9d or xe2x80x9crain fadesxe2x80x9d and are symbolized at reference numerals 14 and 16.
Referring to FIG. 2, an earth station equipped for receiving fundamentally is comprised of an antenna 18, a low-noise amplifier/converter function 20, and a demodulator function 22. The demodulator""s primary function (xe2x80x9cdemodulationxe2x80x9d) is to convert the received modulated signals to data output 24. If the earth station also has transmitting capability, the demodulator function might be included in a modem, and the low-noise amplifier/converter function might be included in a transceiver.
The quality of the received signal is defined by the signal-to-noise ratio. In a digital communications link, signal-to-noise ratio may be normalized to the data rate and unit noise bandwidth, and is then referred to by the quantity Eb/No, where Eb is a signal energy per data bit and No is a noise power spectral density.
Because fades affect both the signal strength and the total noise at the receiving earth station, variations in the uplink and downlink fade attenuations (Au and +Ad, respectively) cause variations in Eb/No. In particular, if the power of the signal at the transmitting station is held constant, during uplink or downlink fades, the Eb/No at the receiving station will degrade.
This degradation might then be compensated for by increasing the power of the signal at the transmitting earth station during a fade event. However, it is often an overriding requirement that the power of each signal at the satellite transponder output must not exceed a maximum value authorized by the satellite operator. Commonly this maximum value is approximately equal to the nominal operating power.
Therefore, the fading on the uplink path may be compensated by increasing transmit power, but fading on the downlink path should not be compensated by increasing transmit power. This imposes a requirement on the compensation algorithm that it must use some means to independently assess the uplink and downlink fade conditions or otherwise maintain constant power at the transponder output.
This is traditionally done in several ways:
a. The uplink earth station is equipped with an additional receiver which measures a constant-power beacon signal provided on the satellite. Measurement of this beacon signal strength is a direct indication of uplink fade, after calibration for frequency difference, due to the known frequency response of the fade medium,
b. The uplink earth station is equipped with an additional receiver which measures the strength and/or signal quality of its own uplinked signal,
c. Each earth station in a geographically-dispersed network reports the received signal quality and/or strength to the uplink earth station (directly, or indirectly via a management earth station). Under the assumption that fading is likely to happen at only a small proportion of the stations, the identity of the station experiencing an uplink fade can be deduced and the uplink and downlink fade independently quantified; and
d. Combinations of the above.
Methods a and b require additional equipment beyond what is necessary to implement the communications link. Method c is not viable in a network of two stations, or if fade occurs simultaneously at a significant proportion of the population of stations in the network.
As indicated above, in order for an earth station to compensate for atmospheric fading by adjusting transmit power while preventing accidental overload of the satellite transponder, it is necessary to separately derive the uplink and downlink fade attenuation values. The invention is directed to a method of deriving and using this information using the same signals and equipment commonly used to implement the communications link.
In accordance with one aspect of the invention, a method is provided for compensating for atmospheric fading in a communication system wherein communication signals are exchanged between first and second earth stations via a satellite link, without increasing power of the satellite link. The method comprises determining at one or both of the earth stations, the signal power of a received signal and a signal-to-noise ratio of the received signal, calculating the difference in noise power spectral density in the received signal from the noise power spectral density under clear sky conditions, calculating the downlink attenuation; determining the uplink attenuation present at a transmitting one of the earth stations and commanding the transmitting one of the earth stations to increase its transmit power by an amount to compensate for the uplink attenuation.
In accordance with another aspect of the invention, there is provided a system for carrying out the foregoing method.